The present invention relates to an electrostatographic reproduction methods and machines.
In this specification, the expression "electrostatographic reproduction machine" refers to a machine for producing one or more prints or copies from at least one electrostatic latent image. The expression "electrostatographic member" refers to a member for producing and utilizing an electrostatic latent image.
The production of an electrostatic latent image may be carried out in various ways, as is well known. The basic and most conventional electrostatographic process or method id described in U.S. Pat. No. 2,297,691. This method involves producing a uniform electrostatic charge on a photoconductive insulating layer. In practice, it is possible for the insulating layer to have a protective overlayer or other overlayer known in the art of xerography. The charged layer is exposed to imaging radiation (especially light) to discharge selectively the photoconductive layer to form the electrostatic latent image. The latent image may then be developed in any known way. Examples of known development methods, for example, are "cascade development" described in U.S. Pat. No. 2,221,776; and "magnetic brush development" described in U.S. Pat. No. 2,874,063. Another example of a known development method is a liquid development method described in U.S. Pat. No. 3,084,043. In this method, development is carried out with a polar liquid developer. Such a developer is stable, i.e., it will respond to an electrostatic field as a homogeneous unit without separation of the components of the liquid developer. As described in U.S. Pat. No. 3,084,043, the polar liquid developer is applied by a rotatable member having a plurality of raised portions defining a substantially regular patterned surface and a plurality of portions depressed or sunken below the raised portions. The liquid developer is present in the depressed portions and is doctored by a doctor blade.
In present electrostatographic machines the photoconductive layer is charged as far as practically possible uniformly along its length. In such machines imaging radiation is normally directed through an optical system from an original document to be copied. Conventional or even especially designed optical systems exhibit an inherent fall-off of efficiency in the optical components at their extreme fields of view. In well known automatic electrostatographic machines, it is usual to move either the original to be copied or the photoconductive layer during the step of imaging to provide a scanning operation. The radiation is directed through a slit or aperture which regulates the time of exposure. Present day machines often compenstate for fall-off of efficiency by providing a butterfly type aperture in the optical system in a plane at right angles to the line of scanning. A butterfly type aperture is wider at its extremities so as to allow more radiation in the extremes of the field of view than in a central region, thereby correcting for the lower light intensity. With some limitations this compensation has proved generally satisfactory. Copending application Ser. No. 654,301, filed on Feb. 2, 1976 suggests the use of a differentially charged photoconductive layer as a means for compensating for fall-off, the differential charge being created by a uniform charging step followed by a non-uniform discharge step in which the photoconductor is exposed to a non-uniform radiation source.